Thiazole compounds as additives in electrolyte solutions in electrochemical cells and batteries

ABSTRACT

The present invention relates to an electrolyte solution comprising at least one solvent as component A, at least one electrolyte as component B and from 0.1 to 20% by weight, based on the total electrolyte solution, of at least one heteroaromatic compound of the general formula (I) as component C, the use of such a compound in electrolyte solutions, the use of such an electrolyte solution in an electrochemical cell or for metal plating, and also electrochemical cells comprising a corresponding electrolyte solution.

This application is a divisional of U.S. application Ser. No.12/966,510, filed Dec. 13, 2010, which claimed priority to EuropeanPatent Application No. 09179174.9, filed Dec. 15, 2009, of which all ofthe disclosures are incorporated herein by reference in theirentireties.

DESCRIPTION

The present invention relates to an electrolyte solution comprising atleast one solvent as component A, at least one electrolyte as componentB and from 0.1 to 20% by weight, based on the total electrolytesolution, of at least one heteroaromatic compound of the general formula(I) as component C, the use of such a compound in electrolyte solutions,the use of such an electrolyte solution in an electrochemical cell orfor metal plating, and also electrochemical cells comprising acorresponding electrolyte solution.

Electrolyte solutions for use in electrochemical cells or in metalplating are already known from the prior art.

Jeong et al., Electrochem. Comm. 10 (2008), 635 to 638, disclose amethod of avoiding the formation of dendritic lithium crystals inconcentrated electrolyte solutions. The electrolyte solutions mentionedin this document comprise appropriate lithium salts as electrolytes.Furthermore, the dependence of the formation of corresponding crystalson the concentration of the lithium salt LiN(SO₂C₂F₅)₂ is examined.

Zhang et al., J. of Powersources 162 (2006), 1379 to 1394, disclosevarious additives for electrolyte solutions which can be used in lithiumion batteries. Organic compounds, for example, heterocyclic compounds,carbonates with unsaturated compounds, aromatic compounds havingunsaturated substituents, and also phosphorus- or silicon-comprisingmolecules are proposed as additives. Boron-comprising complexes ofdicarboxylic acids are mentioned as further suitable compounds.

The electrolyte solutions of the prior art are capable of improvement inrespect of, for example, avoidance of the formation of lithium dendritesduring the charging and discharging cycles. Furthermore, electrolytesolutions which have a wide electrochemical window and are stable towardhighly reactive lithium should be provided.

It is accordingly an object of the present invention to provide anelectrolyte solution by means of which the formation of lithiumdendrites during the charging and discharging cycles can be avoided.Furthermore, the electrolyte solution should have advantageousproperties for use in rechargeable batteries or in metal plating.

These objects are achieved by an electrolyte solution comprising atleast one solvent as component A, at least one electrolyte as componentB and from 0.1 to 20% by weight, based on the total electrolytesolution, of at least one compound of the general formula (I),

as component C, where X¹, X², X³, R² and m have the following meanings:

-   -   X¹, X² are each, independently of one another, N or CH,    -   X³ is NR¹, O or S,    -   m is an integer from 0 to 4,        where at least two of X¹, X² and X³ are heteroatoms selected        from among N, O and S,    -   R¹ is selected from the group consisting of hydrogen, linear or        branched, substituted or unsubstituted C₁-C₁₂-alkyl which may be        interrupted by at least one heteroatom, substituted or        unsubstituted C₃-C₁₂-cycloalkyl which may be interrupted by at        least one heteroatom, substituted or unsubstituted C₅-C₂₀-aryl,        substituted or unsubstituted C₅-C₂₀-heteroaryl, and    -   the radicals R² are selected independently from the group        consisting of linear or branched, substituted or unsubstituted        C₁-C₁₂-alkyl which may be interrupted by at least one        heteroatom, substituted or unsubstituted C₃-C₁₂-cycloalkyl which        may be interrupted by at least one heteroatom, substituted or        unsubstituted C₅-C₂₀-aryl, substituted or unsubstituted        C₅-C₂₀-heteroaryl, halides and functional groups having        electron-pulling character.

Furthermore, the objects are achieved by the use of a compound of thegeneral formula (I) in electrolyte solutions, by the use of the electronsolution mentioned in an electrochemical cell or for metal plating andby an electrochemical cell comprising the electrolyte solution of theinvention.

The electrolyte solution of the invention comprises at least one solventas component A. The at least one solvent present in the electrolytesolution of the invention is known per se to those skilled in the artand is preferably a nonaqueous solvent. In a particularly preferredembodiment, component A is an organic solvent.

In a particularly preferred embodiment, the at least one solvent isselected from the group consisting of carbonates, lactones, heterocycliccompounds, esters, ethers, aromatic compounds and mixtures thereof.

Carbonates which are particularly preferred according to the inventionare, for example, selected from the group consisting of ethylenecarbonate, propylene carbonate, dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC) and mixtures thereof.

Further suitable solvents are lactones, for example gamma-butyrolactone,gamma-valerolactone and mixtures thereof.

Esters which can be preferably used according to the invention areselected, for example, from among aliphatic esters of monocarboxylicacids having, for example, from 1 to 12 carbon atoms, in particularethyl acetate, ethyl propionate, methyl butyrate, ethyl butyrate, methylformate and mixtures thereof.

Ethers which are suitable for the purposes of the invention can belinear or cyclic. Linear ethers are, for example, selected from thegroup consisting of methylene dimethyl ether (DMM), ethylene dimethylether (DME), ethylene diethyl ether (DEE) and mixtures thereof. Cyclicethers which are particularly suitable according to the invention are,for example, selected from the group consisting of tetrahydrofuran(THF), 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane,2-methyl-1,3-dioxolane and mixtures thereof.

Aromatic compounds which are preferred according to the invention are,for example, selected from the group consisting of benzene, toluene,chlorobenzene, xylene and mixtures thereof.

In a particularly preferred embodiment, a mixture of at least two of thecompounds mentioned is used as solvent in the electrolyte solution ofthe invention.

Very particular preference is given to using a mixture of at least onecyclic carbonate, for example ethylene carbonate, and at least onelinear carbonate, for example, diethyl carbonate, as component A. Themixing ratio in the mixtures of at least two of the solvents mentionedwhich can preferably be used according to the invention can be variedwithin a wide range. The mixing ratio can be, for example, from 1:100 to100:1, preferably from 1:20 to 20:1, particularly preferably from 1:5 to5:1.

At least one electrolyte is present as component B in the electrolytesolution of the invention.

According to the invention, all electrolytes known to those skilled inthe art are suitable as component B in the electrolyte solutions of theinvention.

Particular preference is given to electrolytes which have a sufficientlyhigh solubility in the solvents mentioned to give suitable electrolytesolutions. Electrolytes which dissolve completely in the nonaqueoussolvent of the electrolyte solution of the invention and dissociatecompletely are particularly suitable. Furthermore, the solvated ionsshould have high mobility in the electrolyte solution. For use of theelectrolyte solution of the invention in electrochemical cells inparticular, the corresponding anion of the electrolyte used should bestable toward oxidative decomposition at the cathode. Furthermore, theanion should be inert to the solvents used. In addition, both parts,viz. the cationic part and the anionic part, of the electrolyte usedshould be inert to the further components.

In the electrolyte solution of the invention, preference is given tousing an electrolyte whose cation is lithium as component B. Lithium ispreferably present as cation in the oxidation state+1.

The anion of the electrolyte used according to the invention ascomponent B is selected so that the abovementioned requirements are met,in particular dissociability, inertness and stability. Particularlypreferred anions of the electrolyte are, for example, selected from thegroup consisting of halides, for example fluoride, chloride, bromide,iodide, oxides, hydroxides, anionic boron, phosphorus and/or arseniccomplexes, triflates, imides, perchlorates and combinations thereof.

In the electrolyte solution of the invention, particular preference isgiven to using at least one electrolyte selected from the groupconsisting of lithium halides, in particular lithium chloride LiCl orlithium fluoride LiF, lithium oxide Li₂O, lithium salts of boron,phosphorus, arsenic complexes, in particular lithiumtetrafluoroboranate, lithium hexafluorophosphate LiPF₆, lithiumhexafluoroarsenate, lithium perchlorate, lithium triflate, lithiumimide, lithium bis(trifluoromethanesulfonyl)imide and derivativesthereof and mixtures as component B.

Very particular preference is given to using lithium hexafluorophosphateLiPF₆ as electrolyte (component B) in the electrolyte solution of theinvention.

The at least one electrolyte is generally present in the electrolytesolution of the invention in an amount which ensures a sufficiently highconductivity of the electrolyte solution. For example, the at least oneelectrolyte is present in a concentration of from 0.1 to 10 mol/l,preferably from 0.2 to 2 mol/l, very particularly preferably from 0.5 to1.5 mol/l, in the electrolyte solution of the invention.

For the other electrolytes mentioned, the corresponding percentages byweight can be calculated by a person skilled in the art.

The electrolyte solution of the invention comprises from 0.1 to 20% byweight, preferably from 0.1 to 5.0% by weight, particularly preferablyfrom 1.0 to 3.0% by weight, in each case based on the total electrolytesolution, of at least one compound of the general formula (I) ascomponent C,

where X¹, X², X³, R² and m have the following meanings:

-   -   X¹, X² are each, independently of one another, N or CH,    -   X³ is NR¹, O or S,    -   m is an integer from 0 to 4,        where at least two of X¹, X² and X³ are heteroatoms selected        from among N, O and S,    -   R¹ is selected from the group consisting of hydrogen, linear or        branched, substituted or unsubstituted C₁-C₁₂-alkyl which may be        interrupted by at least one heteroatom, substituted or        unsubstituted C₃-C₁₂-cycloalkyl which may be interrupted by at        least one heteroatom, substituted or unsubstituted C₅-C₂₀-aryl,        substituted or unsubstituted C₅-C₂₀-heteroaryl, and    -   the radicals R² are selected independently from the group        consisting of linear or branched, substituted or unsubstituted        C₁-C₁₂-alkyl which may be interrupted by at least one        heteroatom, substituted or unsubstituted C₃-C₁₂-cycloalkyl which        may be interrupted by at least one heteroatom, substituted or        unsubstituted C₅-C₂₀-aryl, substituted or unsubstituted        C₅-C₂₀-heteroaryl, halides and functional groups having        electron-pulling character.

In a preferred embodiment, X¹ is N.

In a further preferred embodiment, X² is CH.

In a further preferred embodiment, X³ is S or O, particularly preferablyS.

In a particularly preferred embodiment, X¹ is N, X² is CH and X³ is S inthe compound of the general formula (I).

In the general formula (I), m indicates the number of substituents R²located on the aromatic ring. m is preferably 0, 1, 2, 3, or 4, veryparticularly preferably 0, 1 or 2, in particular 0.

In a preferred embodiment, the radicals R² are selected independentlyfrom the group consisting of linear or branched, substituted orunsubstituted C₁-C₁₂-alkyl, preferably C₁-C₆-alkyl, particularlypreferably C₁-C₃-alkyl, which may be interrupted by at least oneheteroatom, for example methyl, ethyl, propyl, substituted orunsubstituted C₅-C₂₀-aryl, for example phenyl or naphthyl, andfunctional groups having electron-pulling character, for example groupsselected from the group consisting of halides, for example fluoride.

If present, R¹ is preferably selected from the group consisting ofC₁-C₆-alkyl, particularly preferably C₁-C₃-alkyl.

Substituents which may be present on R¹ and/or R² are, according to theinvention, selected independently from the group consisting of linear orbranched, substituted or unsubstituted C₁-C₁₂-alkyl which may beinterrupted by at least one heteroatom, substituted or unsubstitutedC₃-C₁₂-cycloalkyl which may be interrupted by at least one heteroatom,substituted or unsubstituted C₅-C₂₀-aryl, substituted or unsubstitutedC₅-C₂₀-heteroaryl, halides and functional groups, for example CN, andmixtures thereof.

In a very particularly preferred embodiment, X¹, X², X³ and m have thefollowing meanings:

-   -   X¹ is N,    -   X² is CH,    -   X³ is S,    -   m is 0,        i.e. particular preference is given to using benzothiazole of        the formula (Ia) as compound of the general formula (I) as        component C

Therefore, a very particularly preferred embodiment of the presentinvention provides an electrolyte solution according to the invention inwhich benzothiazole is present as compound of the general formula (I).

In the particularly preferred embodiment in which the electrolytesolution of the invention consists exclusively of the components A, Band C, the amounts of the components A, B and C add up to 100% byweight.

In a very particularly preferred embodiment, the present inventionprovides an electrolyte solution comprising a mixture of at least onecyclic carbonate, for example ethylene carbonate, and at least onelinear carbonate, for example diethyl carbonate, as component A, lithiumhexafluorophosphate LiPF₆ as component B and benzothiazole as componentC, in each case in the amounts indicated above.

The present invention also provides for the use of a compound of thegeneral formula (I) in electrolyte solutions. In particular, the presentinvention provides the stated use, with the compound of the generalformula (I) being used in electrochemical cells.

As regards the preferred embodiments of the compound of the generalformula (I), what has been said above applies. Particular preference isgiven to using benzothiazole as compound of the general formula (I).

Electrochemical cells, in particular their structure and the componentscomprised, are known per se to those skilled in the art. The compound ofthe general formula (I) is particularly preferably used in batteries, inparticular in rechargeable batteries. The general structure of these isdescribed, for example, in “Lithium ion batteries: Science andTechnologies, Gholam-Abbas Nazri, Gianfranco Pistoia, 2009”. Theseelectrochemical cells generally comprise, apart from the electrolyte, atleast one anode, for example a lithium anode, a cathode, for example acopper cathode, lithium and also appropriate insulation elements andconnections.

Furthermore, the present invention provides the use according to theinvention in which the compound of the general formula (I) is used formetal plating.

Processes for metal plating are known per se to those skilled in the artand are described, for example, in “Praktische Galvanotechnik” by P. W.Jelinek, Eugen G. Lenze Verlag, 2005. Use of the compound of the generalformula (I) enables, for example, lithium to be applied to anappropriate substrate, for example a copper substrate.

Furthermore, the present invention provides for the use of theelectrolyte solution of the invention in an electrochemical cell or formetal plating. Preferred embodiments of the electrolyte solution of theinvention and details of electrochemical cells or of metal plating havebeen mentioned above.

FIGURES

FIG. 1 shows a cyclic voltammogram using 3% by weight of benzothiazolein an EC/DEC electrolyte (Example 1). The current in mA is plotted onthe y axis, and the potential in mV is plotted on the x axis.

FIG. 2 shows the cyclability of an Li/NCA cell (Example 3). The capacityin mAh/g is plotted on the y axis, and the cycle number is plotted onthe x axis.

In this figure:

solid line: CC for electrolyte without benzothiazole,

broken line: CC for electrolyte with benzothiazole

solid line with triangles: DC for electrolyte without benzothiazole

broken line with triangles: DC for electrolyte with benzothiazole

Here, CC is the “charging current”, i.e. the current applied forcharging the battery. DC is the “discharging current”, i.e. the currentwhich is applied during discharging.

FIG. 3 shows the cyclability of a C/NMC cell (Example 4). The capacityin mAh/g is plotted on the y axis, and the cycle number is plotted onthe x axis.

In this figure:

solid line: CC for electrolyte

solid line with triangles: DC for electrolyte

For definitions of CC and DC, see above.

EXAMPLES Example 1

Determination of the Electrochemical Window by Cyclic Voltammetry (CV)

The experiment is carried out in a plastic-coated glass vessel. Theworking electrode (WE) is a platinum ring electrode having an area of0.04 cm², and the reference electrode (RE) is lithium metal and islocated in the vicinity of the working electrode. The counterelectrode(CE) is a lithium plate. The electrolyte solution used is a mixture ofethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of 1:3(volume). 5% by weight of benzothiazole are added to this electrolytesolution. The electrolyte is LiPF₆, one molar in the stated solution.The scan rate employed is 20 mV/s in the range from −0.5 V to 5 Vrelative to Li/Li+. The electrochemical window at 4.8 V is shown in FIG.1.

Example 2

Lithium Plating and Morphology

The electrochemical lithium plating experiment is carried out in aplastic-coated glass vessel. Counterelectrode and working electrode arelithium plates having an area of 3.0 cm². Lithium metal is used asreference electrode. The working electrode is supplied with a constantcathode current of 1 mA/cm². The electrolyte composed of ethylenecarbonate and diethyl carbonate (1:3 v/v) with 5% by weight ofbenzothiazole and a control electrolyte are tested under identicalconditions.

The controlled experiment gives a very loose lithium deposit with verypoor adhesion to the plate. The experiment using benzothiazole asadditive gives a very dense, fine deposit with good adhesion. Thecurrent efficiency is calculated as 98%.

Example 3

Cell Cycle

In a Swagelok 3-electrode cell, lithium foil is used as anode on acopper foil as current collector. NCA (Li—Ni—Co oxide) coated with analuminum current collector is used as cathode material; lithium is onceagain used as reference electrode. The cell is assembled under an argonatmosphere in a glove box, with all components having been driedovernight at 120° C. in a vacuum oven. The cell is cycled between 3 Vand 4.2 V below 1 C. Electrolytes with and without benzothiazole aretested. FIG. 2 shows that benzothiazole improves the cyclability of thecell.

In a further test, NMC (Li—Ni—Mn—Co oxide) is used as cathode materialand graphite is used as anode material. The Swagelok setup and the testconditions are retained. FIG. 3 shows the stable performance in thepresence of benzothiazole, which shows that benzothiazole does not haveany negative effect on the graphite anode.

1. A process for metal plating, comprising plating metal using anelectrolyte solution comprising at least one solvent selected from thegroup consisting of a carbonate, heterocyclic compound, ester, ether,aromatic compound and a mixture thereof, as component A, at least oneelectrolyte as component B and from 0.1 to 20% by weight, based on thetotal electrolyte solution, of at least one compound of the formula (I),

wherein X¹ is N, X² is CH, X³ is NR¹, O or S, m is an integer from 0 to4, R¹ is selected from the group consisting of hydrogen, linear orbranched, substituted or unsubstituted C₁-C₁₂-alkyl which may beinterrupted by at least one heteroatom, substituted or unsubstitutedC₃-C₁₂-cycloalkyl which may be interrupted by at least one heteroatom,substituted or unsubstituted C₅-C₂₀-aryl, and substituted orunsubstituted C₅-C₂₀-heteroaryl, and the radicals R² are selectedindependently from the group consisting of linear or branched,substituted or unsubstituted C₁-C₁₂-alkyl which may be interrupted by atleast one heteroatom, substituted or unsubstituted C₃-C₁₂-cycloalkylwhich may be interrupted by at least one heteroatom, substituted orunsubstituted C₅-C₂₀-aryl, substituted or unsubstitutedC₅-C₂₀-heteroaryl, halides and functional groups having electron-pullingcharacter.
 2. The process of claim 1, wherein the at least oneelectrolyte, which is the component B is selected from the groupconsisting of lithium halide, lithium oxide, lithium salt of boron,phosphorus, arsenic complex, lithium perchlorate, lithium triflate,lithium imide, lithium bis(trifluoromethanesulfonyl)imide, derivativethereof and a mixture thereof.
 3. The process of claim 1, whereinbenzothiazole is present as a compound of the formula (I).
 4. Theprocess of claim 1, wherein m is an integer from 1 to
 4. 5. The processof claim 1, wherein the content of the at least one compound of theformula (I) is from 3 to 20 weight % based on the total electrolytesolution.